WASHINGTON - The first study of a possible Ebola vaccine is underway in Africa: University of Maryland researchers say three health care workers in Mali received the experimental shots.

The U.S. government developed the vaccine. First-stage human safety testing began last month in the U.S. and Britain. University of Maryland researchers announced the Mali study Thursday as part of that broader safety testing and aim to enrol 40 health workers there.

The Ebola outbreak is in Liberia, Sierra Leone and Guinea. Mali hasn't had any cases but borders Guinea, and health authorities there are on guard.

Researchers say early safety tests should be done in Ebola-free countries to avoid complicating factors. If the vaccine appears safe, larger trials could be done in the outbreak zone early next year.